1. Field of the Invention
The present invention relates generally to systems for converting between reciprocal and rotary motion. In another aspect, the invention concerns barrel-type engines having an elongated drive shaft that is rotated by a plurality of pistons symmetrically spaced around the shaft and reciprocating generally parallel to the axis of rotation of the shaft. In still another aspect, the invention concerns engines employing a non-hydrocarbon-based fluid as a piston lubricant. In a further aspect, the invention relates to parallel, double-acting Stirling engines employing an expandable and contractible working fluid to drive double-ended pistons. In a still further aspect, the invention concerns Stirling engines which employ heat exchangers that are integral with the cylinder assemblies that house the pistons. In a yet further aspect, the invention concerns systems for varying the power output of Stirling engines.
2. Discussion of Prior Art
Many conventional mechanical devices require reciprocal motion to be converted to rotary motion (e.g., engines) or rotary motion to be converted to reciprocal motion (e.g., pumps). An often-employed system for converting between rotary and reciprocal motion involves the use of a crank arm having a first end coupled to a linearly reciprocating piston and a second end coupled to a rotating crank shaft at a location eccentric to the axis of rotation of the crank shaft. Such an arrangement can be inefficient and can produce excessive vibration and noise. Further, such a system can impart a bending moment on the rotating crank shaft, thereby requiring a larger crank shaft in order to minimize the risk of failure due to fatigue.
As an alternative to systems using crank arms to convert between rotary and reciprocal motion, several crankless systems have been developed. These crankless systems typically employ a swash plate/roller arrangement. In such a arrangement the swash plate is coupled to a drive shaft for rotation therewith and the roller contacts at least one curved cam surface of the swash plate. The roller is coupled to a linearly reciprocating piston so that when the swash plate is rotated, the roller rolls on the curved cam surface, thereby causing the piston to move linearly. Alternatively, when the piston reciprocates linearly, the roller presses on the curve cam surface, thereby causing the swash plate and the drive shaft to rotate. Such prior art swash plate/roller systems, however, typically cause bending stresses on the drive shaft. Further, such systems have typically produced excessive noise and vibration due to their lack of dynamic balance.
Stirling engines (i.e., external combustion engines) have existed for years but have not been widely commercially implemented. Stirling engines typically operate by heating and cooling an expandable and contractible working fluid in a working fluid chamber to thereby drive reciprocating pistons. A potentially very efficient Stirling engine is known as a xe2x80x9cparallelxe2x80x9d form of the Franchot engine, described and illustrated in Principles and Applications of Stirling Engines, by C. D. West, 1986, pp 64-65, the disclosure of which is incorporated herein by reference. In such a xe2x80x9cparallelxe2x80x9d Stirling engine, an expansion (i.e., hot) cylinder and a compression (i.e., cold) cylinder, both containing respective double-ended pistons, cooperate with the compressible working fluid to drive the double-ended pistons. One significant advantage of the parallel Stirling arrangement is that the entire expansion cylinder (including both ends of the cylinder) is heated while the entire compression cylinder is cooled. This results in the virtual elimination of thermal shuttle losses typically experienced in serially connected Stirling engines comprising individual cylinders which each have a hot end and a cold end. However, a significant disadvantage of prior art parallel Stirling engines is the difficultly in maintaining a proper working fluid seal when a reciprocating piston rod coupled to the piston extends through an end portion of the cylinder wall that defines the working fluid chamber.
Stirling engines typically employ non-lubricated teflon piston rings to prevent the escape of the working fluid from the working fluid chambers. The main reason teflon rings are used rather than more conventional metallic piston rings is that metallic rings require conventional hydrocarbon-based lubricants to ensure efficient extended operation of the engine. However, using a conventional hydrocarbon-based lubricant in conjunction with a metallic piston ring will inevitably result in some lubricant passing from the lubricant holding chamber on one side of the piston into the working fluid chamber on the other side of the piston. The presence of even trace amounts of conventional hydrocarbon-based engine lubricants in the working fluid chamber of a Stirling engine is highly undesirable because these lubricants, when entrained in the working fluid, can irreversibly contaminate the regenerator of the Stirling engine. Thus, conventional engine lubricants can not be effectively employed to lubricate the pistons of a conventional Stirling engine. However, the solution of employing non-lubricated teflon piston rings in a Stirling engine has its own drawbacks. In particular, the physical properties of teflon (particularly its low melting point) place an upper temperature limit at which the piston cylinder can be maintained without damaging the teflon ring. This problem is especially pronounced in parallel Stirling engines employing double-ended pistons because the piston ring must be located proximal the working fluid chambers at each end of the pistons. Thus, a significant disadvantage of using teflon piston rings in a parallel Stirling engine is that the working fluid can not be heated to its optimum temperature without damaging the teflon piston rings.
A further disadvantage of prior art Stirling engines is the inefficiency of locating the heat exchangers remotely from the expanding and compression cylinders. Although spacing the heat exchangers from the expansion and compression cylinders allows for adequate heat exchange between the working fluid and the heat transfer fluid (i.e., the heating or cooling source), such a configuration does not allow for heat to be conducted directly from the physical structure of the heat exchanger to the physical structure of the cylinder assembly.
A still further disadvantage of prior art Stirling engines is their inability to rapidly vary the power output of the engine.
Responsive to these and other problems, it is an object of the present invention to provide an apparatus for converting between rotary and reciprocal motion without imparting a significant bending moment on a rotating drive shaft of the apparatus.
A further object of the present invention is to provide a dynamically balanced apparatus for converting between rotary and reciprocal motion.
A still further object of the present invention is to provide an apparatus for converting between rotary and reciprocal motion that has a more compact and robust construction than prior art devices.
An even further object of the present invention is to provide a parallel Stirling engine which employs a unique piston rod arrangement wherein the piston rod does not extend through a wall that defines the working fluid chamber.
Still a further object of the present invention is to provide a Stirling engine having a heat exchanger which is integral with the cylinder assembly to thereby allow heat to be directly conducted from the physical structure of the heat exchanger to the physical structure of the cylinder assembly.
Another object of the present invention is to provide a system which lubricates the pistons of a Stirling engine without causing contamination of the regenerator.
Still another object of the present invention is to provide a Stirling engine having the ability to rapidly vary the power output of the engine.
It should be noted that the above-listed objects need not all be accomplished by the invention claimed herein and other objects and advantages of this invention will be apparent from the following description of the invention and appended claims.
In accordance with one embodiment of the present invention, a motion converting apparatus is provided. The motion converting apparatus generally comprises an elongated shaft, a pair of spaced-apart cam disks, a reciprocating piston, and a pair of cam engagement bearings. The elongated shaft is adapted for rotation on a shaft axis. The cam disks are coupled to the shaft for rotation therewith and each present an inwardly facing curved cam surface. The piston is positioned generally between the inwardly facing cam surfaces and is adapted for linear reciprocal motion in a direction at least substantially parallel to the shaft axis. The can engagement bearings are coupled to the piston for reciprocal motion therewith. Each of the bearings rollingly contact a respective cam surface. The piston is positioned generally between the bearings.
In accordance with another embodiment of the present invention, an engine is provided. The engine generally comprises an elongated drive shaft, a pair of spaced apart cam disks, a plurality of reciprocating pistons, a pair of bearing assemblies for each piston, and a plurality of piston rod assemblies. The elongated drive shaft is adapted for rotation on a shaft axis. The cam disks are each coupled to the shaft for rotation therewith and each present a curved cam surface. The curved cam surfaces face generally inwards towards one another. The reciprocating pistons are positioned between the cam surfaces and are adapted for linear reciprocal motion in a direction at least substantially parallel to the shaft axis. The pistons are spaced generally symmetrically around the shaft axis. One pair of bearing assemblies is associated with each piston. Each bearing assembly comprises a housing and roller bearings supported for rotation relative to the housing. Each roller bearing in each bearing assembly contacts a respective one of the cam surfaces. Each of the pistons is positioned generally between the pair of bearing assemblies associated with that piston. Each of the piston rod assemblies couples one of the pistons to the pair of bearing assemblies associated with that piston.
In accordance with a further embodiment of the present invention, an engine generally comprising a housing, a cylinder assembly, and a piston is provided. The housing at least partly defines an inner chamber. The cylinder assembly is disposed in the inner chamber and at least partly defines an internal cylinder chamber disposed generally within the cylinder assembly and an external chamber disposed generally outside the cylinder assembly. The internal cylinder chamber and external chamber are at least substantially fluidically isolated from one another. The cylinder assembly presents an internal cylinder wall which at least partly defines the internal cylinder chamber. The piston is shiftably disposed in the internal cylinder chamber and presents a sealing surface at least substantially sealingly contacting the internal cylinder wall. The piston separates the internal cylinder chamber into a working chamber and a piston rod chamber. The working chamber and the piston rod chamber are at least substantially fluidically isolated from one another.
In accordance with a still further embodiment of the present invention, a cylinder assembly for a Stirling engine is provided. The Stirling engine utilizes thermal energy transferred between a working fluid and a heat transfer fluid to generate mechanical energy via a reciprocating piston. The cylinder assembly generally comprises a piston chamber wall, a heat transfer chamber, a heat exchanger, and a thermally conductive wall. The piston chamber wall at least partially defines an internal cylinder chamber. The internal cylinder chamber is adapted to shiftably receive the reciprocating piston. The piston chamber wall is adapted to cooperate with the piston to at least partly define a working fluid chamber of variable volume within the cylinder assembly. The heat transfer chamber is fluidically isolated from the working fluid chamber. The heat exchanger is at least partly disposed in the heat transfer chamber and defines a working fluid passageway fluidically communicating with the working fluid chamber. The heat exchanger is adapted to facilitate the transfer of heat between the heat transfer fluid in the heat transfer chamber and the working fluid flowing through the working fluid passageway. The thermally conductive wall defines at least a portion of the heat transfer chamber and is physically coupled to the piston chamber wall. The thermally conductive wall is operable to conduct heat between the heat transfer chamber and the piston chamber wall.
In yet another embodiment of the present invention, a double-barrel Stirling engine is provided. The double-barrel Stirling engine generally comprises an elongated drive shaft, a pair of spaced-apart inner cam disks, a pair of spaced-apart outer cam disks and a power actuator. The elongated drive shaft is adapted for rotation on a shaft axis. The inner cam disks are coupled to the drive shaft for rotation therewith and each present a generally inwardly facing curved inner cam surface. The outer cam disks are coupled to the drive shaft for rotation therewith and each present a generally inwardly facing curved outer cam surface. The power actuator is operable to rotate the inner and outer cam disks relative to one another.
In accordance with yet still another embodiment of the present invention, a Stirling engine having an expansion piston positioned for linear reciprocal movement in an expansion cylinder and a compression piston positioned for linear reciprocal movement in a compression cylinder is provided. The pistons reciprocate at substantially the same rate. The reciprocal motion of one of the pistons trails the reciprocal motion of the other of the pistons in accordance with a piston phase angle. The Stirling engine generally comprises a first member, a second member, a output member, and means for selectively shifting one of the first or second members relative to the other members. The first member is adapted to be rotated by the expansion piston. The second member is adapted to be rotated by the compression piston. The output member is cooperatively rotated by the first and second members and provides a power output. Selective shifting of the members by the means for selectively shifting causes the piston phase angle to change, thereby varying the power output of the Stirling engine.